Replace specific values in a range with cell value in the same row

Hello all,

Scenario: I have two columns, A & B both filled with values. In column B, there are many cells with "NULL" as their value. I want to write a macro that looks for all instances of "NULL" in the entire range of column B, and replaces them with the value of the cell in the same row as Column A. Thus:

A B
1 NULL
2 Hello
3 NULL

would become

A B
1 1
2 Hello
3 3

Is there any easy way to do this without a for loop, perhaps a built-in vba function that can do this? Thanks everyone.